Servo-assisted mechanical transmissions, which are structurally similar to mechanical manual transmission of the traditional type except for the fact that the clutch pedal and the gear selection lever operated by the driver are replaced by corresponding electrical or hydraulic servo-controls, are increasingly widespread. By using a servo-assisted mechanical transmission, the driver only needs to send the order to shift gear up or down to a transmission control unit and the transmission control unit autonomously shifts gear by operating both on the engine and on the servo-controls associated to clutch and gearbox.
The gear shifting order may be generated either manually, i.e. following a command imparted by the driver, or automatically, i.e. regardless of the driver's action. When the gear shifting order is generated, the transmission control unit drives the servo-control of the clutch to open the clutch so as to mechanically separate a primary shaft of the gearbox from a crankshaft; at the same time, the transmission control unit acts on the engine control unit to temporarily reduce the motive torque supplied by the engine itself.
Once the transmission control unit has verified the opening of the clutch, the transmission control unit drives the gearbox servo-control to disengage the currently engaged gear; when the transmission control unit has verified the gear disengagement, the transmission control unit drives the gearbox servo-control to displace the gearbox control shaft so as to arrange the engagement of the new gear. Once the transmission control unit has verified that the gearbox control shaft has reached the required position, the transmission control unit drives the gearbox servo-control to engage the new gear.
Finally, when the transmission control unit has verified that the new gear has been engaged, the transmission control unit drives the clutch servo-control to close the clutch and make the primary shaft of the gearbox and the crankshaft reciprocally and angularly integral; at the same time, the transmission control unit acts on the engine control unit to restore the motive torque supplied by the engine itself.
The clutch servo-control is generally of the hydraulic type and comprises a single hydraulic actuator for displacing the clutch from the closed position to the open position and vice versa. Generally, the gearbox servo-control is also of the hydraulic type and acts on a gearbox control shaft to impress on the control shaft itself both an axial displacement, i.e. along a central axis, to select the gear range, and a rotation about the central axis to engage and disengage each gear. Consequently, the gearbox servo-control comprises a first hydraulic actuator mechanically coupled to the control shaft to axially displace the control shaft and a second hydraulic actuator mechanically coupled to the control shaft to turn the control shaft.
Each hydraulic actuator comprises at least one actuation chamber delimited on one side by a mobile piston, which slides along the actuation chamber itself and is connected to a control shaft; the actuation chamber is filled with a pressurized fluid to displace the piston along the actuation chamber itself. Solutions have been suggested in which each hydraulic actuator comprises two chambers which are alternatively filled with pressurized fluid to move the piston in the two directions, and solutions have also been suggested in which each hydraulic actuator comprises a single actuation chamber which is filled with the pressurized fluid to displace the piston in a single direction and against the elastic bias generated by a spring which provides for displacing the piston in the opposite direction.
The actuation chamber of a hydraulic actuator normally presents a cylindrical tubular shape and is delimited on one side by the piston and on the opposite side by a plug; the plug is provided with at least one static annular seal and the piston is provided with at least one sliding annular seal. Leakages of the pressurized fluid which is fed to the actuation chamber are always possible through such seals; these leakages are undesired, because they reduce the amount of fluid present in the hydraulic system (thus after a certain time the fluid level in the hydraulic system needs topping up) and may soil both the hydraulic actuator and the components arranged underneath the hydraulic actuator (the leaked fluid tends to move downwards by gravity).